1. Field of the Invention
The present invention relates to a system for optically and acoustically detecting ice and sleet pellets in a remote weather monitoring system in which precipitation has been detected.
2. Description of the Prior Art
The primary purposes of automatic weather identification is to provide for remote monitoring of weather without the necessity for human observation at the weather monitoring site. Such a technique allows precipitation conditions at unmanned airfields to be remotely monitored and reported to the pilots of aircraft in flight who may be using those airfields.
A system has previously been devised which provides for remote monitoring of weather without the necessity for human observation at a weather monitoring site in which the presence of precipitation has been detected optically using a partially coherent light beam source. This system is described in my prior U.S. Pat. No. 4,760,272 entitled OPTICAL PRECIPITATION DETECTION AND IDENTIFICATION SYSTEM USING SCINTILLATION DETECTION issued Jul. 26, 1988. That system operates upon the principal that correct identification of different types of precipitation is possible in a near field region which involves transmission of a partially coherent light beam over a path length which is much shorter than that of prior conventional optical systems.
According to the system of my prior patent a partially coherent light source, such as an infrared light emitting diode, transmits a light beam wherein the product of the distance between the transmitter and receiver and one half the angle of incoherency of the partially coherent light beam source is no greater than about 2.5 mm. The distance between the transmitter and the receiver is on the order of about one meter.
Precipitation passing through the light beam produces a characteristic "signature" in frequency of the output of the optical receiver, depending upon whether the precipitation is rain or snow. If the detected precipitation is rain the output frequency of the detected scintillations is above about one kHz. Snow induced frequencies are primarily below a few hundred Hz. The ratio of the quantity of scintillations in the upper band to the quantity in the lower band is used in my prior system to discriminate between rain and snow. That is, rain is indicated when the ratio of scintillations in the high band to the low band is great. Conversely, snow is indicated where the ratio of scintillations in the high band to the low band is small.
One difficulty with my prior system is that it lacks a provision for identifying the presence of ice pellets, such as sleet and hail. The optical scintillations produced by ice pellets typically generate output frequencies in the higher range, and therefore are inaccurately identified as rain by the electronics employed in my prior system.
The National Weather Service requires human observers to separately categorize solidified precipitation, that is sleet or ice pellets, from both rain and snow. However, thus far it has not been possible to remotely make this distinction in prior optical scintillation detection devices. Accordingly, the individuals remotely monitoring the output of the instrumentation have not been provided with information as to whether the precipitation detected at a remote area is solidified, that is, whether or not it is arriving at ground level in the form of ice pellets as sleet or hail.